One prior art technique for introducing dopants into a silicon wafer is to direct an ion beam along a beam travel path and selectively position silicon wafers to intercept the ion beam. This technique dopes the wafer with controlled concentrations of the ion material.
One example of a commercial ion implanter is the Eaton NV 200 Oxygen Implanter. This prior art ion implanter utilizes an oxygen ion source having a cathode that includes a filament for providing electrons for ionizing oxygen molecules. Electrons emitted by the cathode are accelerated through a region containing oxygen gas in controlled concentrations. The electrons interact with the gas molecules, yielding energy to the molecules which ionizes the molecules. Once ionized, the charged oxygen molecules are accelerated and shaped to form a well-defined oxygen ion beam for silicon wafer implantation. An ion source utilizing a cathode filament is disclosed in U.S. Pat. No. 4,714,834 which issued in the name of Shubaly and which is incorporated herein by reference.
Alternate proposals for ion source construction include the use of a microwave ion source that does not require a cathode or cathode filament. A microwave-powered ion source excites free electrons within an ionization chamber at a cyclotron resonance frequency. Collision of these electrons with gas molecules ionizes those molecules to provide ions and more free electrons within the chamber. These ions are then subjected to an accelerating electric field and exit the chamber in the form of an ion beam.
The theory and operation of a microwave ion source are discussed in two printed publications entitled, "Microwave Ion Source For Ion Implantation" to Sakudo, Nuclear Instruments and Methods In Physics Research, B21 (1987), pgs. 168-177 and "Very High Current ECR Ion Source For An Oxygen Ion Implanter" to Torii, et al., Nuclear Instruments and Methods In Physics Research. B21 (1987), pgs. 178-181. The disclosure of these two printed publications is incorporated herein by reference.
The ion sources disclosed in the two aforementioned printed publications includes an ion chamber surrounded by structure for providing a magnetic field for confining an electron plasma within the ion chamber. The necessity of providing a generally axial magnetic field within the ion producing chamber is recognized. It is a prerequisite for the electron cyclotron resonance effect and reduces the frequency with which electrons impact the walls of the ionization chamber. Such impact not only increases the temperature of the chamber, but also results in inefficient utilization of the microwave energy supplied to the ion source.
The low energy ions which are produced in the region of the plasma chamber where the microwave energy is introduced will drift in spiralling orbits about the magnetic field lines. Therefore, in order to make a large fraction of these ions available for extraction, the magnetic field should remain largely non-divergent until beyond the extraction region of the chamber.
Both references disclose embodiments of an ion generation chamber which have one or more encircling solenoids for creating an axially aligned magnetic field within the ion chamber. For an ion chamber suitable for retrofitting with the aforementioned NV 200 Oxygen Implanter, the use of a solenoid for generation of a uniform magnetic field produces a mis-match in size between the existing implanter and the ion source.
FIG. 13 of the Sakudo reference discloses an alternate system wherein a magnetic coil for providing an axial magnetic field is surrounded by an iron or high permeable metal to provide a magnetic circuit for focusing the magnetic field within the ion chamber. A second proposal shown in FIG. 13 of Sakudo is the use of an iron acceleration electrode at the exit portion of the ion chamber. Sakudo presents data indicating the ion source constructed in accordance with this disclosure has been used in combination with a commercial ion implanter with adequate results.